Speaking on ITV's Peston On Sunday he added: "It is not alarmism. We get tired of people saying we are fear-mongering. This is a clear and present danger to our peace process which needs to be tackled."

Last week the PSNI's Chief Constable George Hamilton warned that any infrastructure at the border would become a target for armed groups and pose a danger to his officers.

"The terrorists only have to be lucky once to get a result with catastrophic consequences," he said.

The EU's chief Brexit negotiator Michel Barnier said on Friday that a UK decision to leave the single market and customs union would make Irish border checks "unavoidable".

He made his comments after Downing Street said the UK would not be in a customs union after Brexit.

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